bike size help

This may seem like a dumb question but I'm buying a new stumpjumper hardtail and I'm wondering what frame size I should get. I am 5' 9'' and have a 32" inseam. Should I go for the 18" or the 19" and why.

Yeah, test ride first. If you're still undecided, I'm 5'9" with a 30" inseam and I ride a 17" Rockhopper. My 6', (30.5" inseam) son rides the same bike. He says it's a little small, but on a technical trail, he likes it that way. It's easier to throw the bike around when it's a tad small (but not too small). Try running a few searches on this forum. You'll find some previous threads about bike size with comments about buying a bike that is on the large side, perfect fit or small side.